Bonding pad structure to minimize IMD cracking

ABSTRACT

A method is disclosed of forming a bonding pad that is immune to IMD cracking. A partially processes semiconductor wafer is provided having all metal levels completed. A blanket dielectric layer is formed over the uppermost metal level. Patterning and etching said dielectric layer horizontal and vertical arrays of trenches are formed passing through the dielectric layer and separating the dielectric layer into cells. The trenches are filled with a conducting material and, after performing CMP, bonding metal patterns are deposited. Wires are bonded onto said bonding metal patters, after which a passivation layer is formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of currently U.S. non-provisionalpatent application Ser. No. 10/916,797, filed Aug. 12, 2004, now U.S.Pat. No. 7,135,395, by Liu et al., titled “Novel Bonding Pad Structureto Minimize IMD Cracking,” which is a continuation of U.S.non-provisional patent application Ser. No. 09/945,432, filed Sep. 4,2001, by Liu et al., now U.S. Pat. No. 6,875,682, titled “Novel Mesh PadStructure to Eliminate IMD Crack on Pad,” the entire contents of whichare expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to semiconductor integratedcircuit processing and more particularly to contact pad structures thatresist intermetal dielectric cracking.

(2) Description of Prior Art

Bonding pads are the interfaces between the integrated circuitscontained in semiconductor chips and the chip package. A large number ofbonding pads is required to transmit power, ground and impute/outputsignals to the chip devices. It is thus important that the bonding padyield be sufficiently high to ensure a high per chip yield.

The general bonding pad structure consists of metal layers, emanatingfrom the terminals of the chip devices, separated by intermetaldielectric (IMD) layers that are often silicon oxide. Metal vias, W isoften used, pass through the IMD layers connecting the metal layers.Wires are bonded to a bonding metal pattern and to the chip packageforming electrical connections between the chip and the package. Apassivation layer covers the surface, except over the bonding sites, toseal the chip from contaminants and for scratch protection.

One mode of failure of the bonding pad relates to the peeling of thewire from the metal pattern due to forces exerted especially during thebonding process. This has been addressed in U.S. Pat. No. 6,002,179 toChan et al., who teach a bonding pad structure with increased peelingresistance and in U.S. Pat. No. 5,731,243 to Peng et al., who show acleaning method to ensure contamination free bonding. Another failuremode that has been observed relates to bonding pad peel back, whereforces during wire bonding may cause a delaminating of one or more ofthe underlying layers. Bonding pad structures that resist bond padpeeling have been disclosed in U.S. Pat. No. 6,025,277 to Chen et al.and in U.S. Pat. No. 5,707,894 to Hsiao.

Another failure mode involves cracking of the IMD. Referring to FIGS. 1a, 1 b, and 1 c, there is shown conventional via hole arrays. Regions 10are IMD oxide layers, and regions 12 are metal filled via holes passingthrough the IMD. Cracks that are observed in the IMD are similar tocracks 22 depicted in FIG. 2. These are cracks that propagate along theIMD layer avoiding the metal of the vias. Once a small crack isinitiated it will, under stresses prevalent in the layer duringprocessing, grow extensively. Approaches to alleviate this cracking ofthe IMD focus on producing IMD layers with low residual stress.Composite silicon oxide layers serve this purpose and are used, such asHDP plus PETEOS layers. However, even with composite silicon oxidelayers to reduce stress, the IMD layer is not strong enough to withstandstresses encountered during chip packaging and IMD cracking is stillobserved.

SUMMARY OF THE INVENTION

It is a primary objective of the invention to provide a bonding padstructure that is immune to IMD cracking, withstanding even the stressesencountered during chip packaging. A novel mesh pad structure isproposed that will increase the bonding pad strength and eliminateextensive cracking of the IMD. Instead of traditional via holes, viatrenches are formed through the IMD, dividing the remaining IMD intosmall cells. After the trenches are filled with metal the metal trenchesenclose the cells. This increases the strength of the bonding pad sothat IMD cracking is less likely to occur. Furthermore, even in theunlikely event of the initiation of an IMD crack, the crack willpropagate no further than the metal trench.

A method is disclosed of forming a bonding pad that is immune to IMDcracking. A partially processed semiconductor wafer is provided havingone to all but one metal levels completed. A blanket dielectric layer isformed over the metal level. Patterning and etching said dielectriclayer horizontal and vertical arrays of trenches are formed passingthrough the dielectric layer and separating the dielectric layer intocells. The trenches are filled with a conducting material and, afterperforming CMP, bonding metal patterns are deposited. Wires are bondedonto said bonding metal patterns, after which a passivation layer isformed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing forming a material part of this description,there is shown:

FIGS. 1 a, 1 b and 1 c, Prior Art, show conventional via patterns.

FIG. 2 depicts a crack in the IMD of a conventional bonding pad.

FIG. 3 shows the basic pattern of the mesh via trenches.

FIG. 4 shows a mesh via trench structure without trench intersection.

FIG. 5 shows a mesh via trench structure with a brick-laying pattern.

FIG. 6 shows a mesh via trench structure with a modified brick-layingpattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates the basic pattern of the novel mesh pad structure.The IMD, 10, is separated into cells by perpendicular arrays of metalfilled via trenches. The array 14 is denoted the vertical array and thearray 16 the horizontal array. The strength of the IMD-via trenchstructure is higher than that of the traditional IMD-via holestructures, such as those depicted in FIGS. 1 a, 1 b and 1 c. Thus,initiation of cracks in the IMD will occur less frequently for the meshpad structure than for traditional structures utilizing via holes, suchas those depicted in FIGS. 1 a, 1 b and 1 c. Furthermore, even in theremote possibility of initiation of a crack in the IMD of a mesh padstructure, the crack could only propagate as far as the metal filledtrench which border the IMD cells. Thus the crack size is limited to beless than about the cell diagonal. In the case of traditional via holepad structures, such as those depicted in FIGS. 1 a, 1 b and 1 c, thecrack can propagate over large distances avoiding metal filled viaholes. The reduced damage in the case of a mesh pad structure ismanifested in substantial improvement of the quality and reliability ofthe bonding pad.

Basic elements of a bonding pad structure consist of metal layers,emanating from the terminals of the chip devices, separated by IMDlayers. Also there is an IMD layer separating the uppermost metal layerfrom a bonding metal pattern that is formed over this IMD layer andthere are metal connectors passing through the IMD layers connecting themetal layers to the bonding metal pattern. Wires are bonded to thebonding metal pattern and to the chip package forming electricalconnections between the chip and the package. A passivation layer coversthe surface, except over the bonding sites, to seal the chip fromcontaminants and for scratch protection.

A mesh via trench pattern can be used between any two levels of metal.However, its crack resistance properties are mostly utilized when usedbetween the uppermost metal layer and the bonding metal pattern. To formthe via trench pattern, a blanket dielectric, layer is first formed overthe uppermost metal layer, using techniques well known to those skilledin the art. This dielectric layer is often silicon oxide. Compositelayers are useful in relieving internal stress in the dielectric, stressthat contributes to cracking in the dielectric layer, and preferredembodiments of the invention utilize such layers. Composite dielectriclayers that are used to relieve internal stress include dual oxidelayers, where, for instance, one of the layers is formed using HDP andthe other using PETEOS, for example, 7000 Angstroms can be depositedusing HDP and 17000 Angstroms using PETEOS.

However, composite dielectric layers do not protect the IMD layers fromcracking as a result of stresses arising during chip packaging. Thisprotection is achieved by the novel mesh pad structures of theembodiments of the invention. In contrast to the traditional bondingpad, in which via holes through the IMD layer are used to provideelectrical connection between the metal layers, in a mesh pad structureelectrical connection is achieved by via trenches. Via trenches areformed using the same well known processes as via holes, except that theshapes of the openings are rectangular-like. Via trench layouts aredesigned to separate the IMD into small cells, which, when the trenchesare filled with metal, are essentially surrounded by metal filledtrenches. For trench widths of between about 0.1 and 0.5 micrometers andfor trench lengths between about 0.1 and 100 micrometers, which alsoprovides the cell dimension, the IMD strength is significantlyincreased, and crack sizes are limited to less than about the celldiagonal. A via trench layout according to preferred embodiments of theinvention in which trenches do not intersect is shown in FIG. 4. Thisform of layout will be referred to as the nonintersecting layout. Arraysof horizontal, 16, and vertical, 14, trenches nearly divide the IMDlayer, 10, into cells though they do not intersect. Trench widths arebetween about 0.1 and 0.5 micrometers and the trench lengths are betweenabout 0.1 and 100 micrometers for trenches in both the vertical andhorizontal arrays. In this layout there is a separation between a trenchand its perpendicular neighbors. An advantage of nonintersecting viatrenches is that there is a tendency toward void formation when fillingan intersection with metal and nonintersecting via trenches avoids thisvoid formation. In this layout the trenches do not fully surround theIMD. However, if the ratio between the spacing of perpendiculartrenches, 24, to the spacing of parallel trenches, 26, is kept small,less then about 1/5, cracks will not propagate much beyond a cell beforebeing stopped by a trench. A spacing of perpendicular trenches greaterthen about 0.1 micrometers is required, however, to avoid overlap.Another trench layout according to preferred embodiments of theinvention is referred to as the bricklaying layout and is depicted inFIG. 5. Here the trenches, 14 and 16, do actually divide the IMD layer,10, into closed cells. However, even though the vertical and horizontaltrench arrays do not completely cross each other, there is, T-shapedoverlap at positions, 18. Void formation still occurs during metalfilling of the trenches at overlaps such at positions 18, however thisis at a reduced frequency as compared with crossing intersections.Trench widths are between about 0.1 and 0.5 micrometers and the trenchseparation is between about 0.1 and 10 micrometers for horizontaltrenches and between about 0.1 and 10 micrometers for vertical trenches.To further reduce the tendency for void formation at the overlaps, atrench layout, denoted as the modified bricklaying layout and shown inFIG. 6, is utilized in other preferred embodiments of the invention.Except for overlap region, 20, the trench layout and dimensions for themodified bricklaying layout are similar to the trench layout anddimension for the bricklayer layout. The overlap region, 20, for themodified bricklaying layout is reduced from that of the overlap region,18, for the bricklaying layout and results in a reduction in voiding.Modified bricklaying overlaps between 0.1 and 1 micrometer of thebricklaying overlap achieve significant reductions in voiding, yetprovide complete enclosure of the IMD in the cells.

Filling of the via trenches with conductive material is accomplished, inpreferred embodiments of the invention, using W plug processes, whichare well known to those versed in the art. Other embodiments of theinvention utilize alternative plug processes, such as Al plug, Cu plug,or silicide plug processes. Following the metal filling of the trenches,chemical/mechanical polishing (CMP), a process well known topractitioners of the art, is used to planarize the surface. Bondingmetal patterns are then deposited, according to procedures well known tothose versed in the art. Wires are bonded to bonding metal patterns anda passivation layer is formed, using processes, for both, that wellknown to those versed in the art.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.

1. A bonding pad, comprising: a semiconductor wafer; a dielectric layerdisposed over said wafer, said dielectric layer having a plurality ofhorizontal and vertical trenches passing therethrough; a conductingmaterial disposed within said horizontal and vertical trenches; metalpatterns disposed on said conducting material; and wires disposed onsaid metal patterns; wherein each of said vertical trenches has a firstlength, a first width, and a longitudinal axis parallel to the firstlength, the first length being greater than the first width; and each ofsaid horizontal trenches has a second length, a second width, and amidpoint bisecting the second length, the second length being greaterthan the second width; and wherein the longitudinal axis of each of thevertical trenches intersects the midpoint of an adjacent horizontaltrench.
 2. The bonding pad of claim 1, wherein said dielectric layer isa material selected from the group consisting of: silicon oxide, siliconnitride and silicon oxynitride.
 3. The bonding pad of claim 1, whereinsaid dielectric layer is a composite of a plurality of dielectriclayers.
 4. The bonding pad of claim 1, wherein the separation betweenneighboring horizontal trenches and neighboring vertical trenches isbetween 0.1 and 10 micrometers.
 5. The bonding pad of claim 1, whereinsaid conducting material is selected from the group consisting of Al, Cuand silicide.
 6. The bonding pad of claim 1, wherein the first andsecond widths are each between 0.1 and 0.5 micrometers.
 7. The bondingpad of claim 1, wherein a separation between neighboring horizontaltrenches is between 0.1 and 10 micrometers and a separation betweenneighboring vertical trenches is between 0.1 and 10 micrometers.
 8. Thebonding pad of claim 1, wherein the first lengths are orientedsubstantially perpendicular to the second lengths.
 9. The bonding pad ofclaim 1, wherein the plurality of vertical and horizontal trenchesseparate the dielectric layer into a plurality of cells, and whereinnone of the vertical or horizontal trenches intersect with each other.10. A bonding pad, comprising: a semiconductor wafer; a dielectric layerdisposed over a metal level of said wafer; a conducting materialdisposed within said dielectric layer, said conducting material forminga plurality of horizontal and vertical trenches; and bonding metalpatterns disposed over top surfaces of said conducting material; whereinsaid vertical trenches each has a first length, a first width, and alongitudinal axis parallel to the first length, the first length beinggreater than the first width; and said horizontal trenches each has asecond length, a second width, and a midpoint bisecting the secondlength, the second length being greater than the second width; andwherein none of said vertical trenches intersects with any of saidhorizontal trenches; and wherein the longitudinal axis of each of thevertical trenches intersects the midpoint of an adjacent horizontaltrench.
 11. The bonding pad of claim 10, wherein said dielectric layeris a material selected from the group consisting of silicon oxide,silicon nitride, and silicon oxynitride.
 12. The bonding pad of claim10, wherein said dielectric layer is a composite of a plurality ofdielectric layers.
 13. The bonding pad of claim 10, wherein theseparation between neighboring horizontal trenches and neighboringvertical trenches is between 0.1 and 10 micrometers.
 14. The bonding padof claim 10, wherein the conducting material is selected from the groupconsisting of Al, Cu, and silicide.
 15. The bonding pad of claim 10,wherein the first and second widths each are between 0.1 and 0.5micrometers.
 16. The bonding pad of claim 10, wherein the separationbetween neighboring horizontal trenches is between 0.1 and 10micrometers and neighboring vertical trenches is between 0.1 and 10micrometers.
 17. The bonding pad of claim 10, wherein the first lengthsare oriented substantially perpendicular to the second lengths.
 18. Thebonding pad of claim 10, wherein the plurality of vertical andhorizontal trenches separate the dielectric layer into a plurality ofcells, and wherein none of the vertical or horizontal trenches intersectwith each other.